As the number of users of cellular radio systems increases and rapid data transmission becomes more and more common in the systems, it becomes essentially important to increase the capacity of the system by improving the system performance.
One way to increase the capacity is to use one or more adaptive antenna arrays instead of sector antennas. An antenna array comprises at least two antenna elements correlated with each other, the signals of which can be processed independently from each other. By weighting the signals of the antenna elements of an adaptive antenna array it is possible to form radiation patterns, the signal power of which can be allocated user-specifically to each unit of user equipment. Thus, the signal related to the radiation pattern transmitted user-specifically also comprises user-specific coding.
In an ideal case, the transmitted signal and the received signal are identical. In practice, however, this is not always the case, but for instance fading and distortion of the signal occurs on the radio path. Particularly in the case of user equipment in a moving state, the used radio channel can change as the function of time and during each individual connection.
The changes in the signal caused by the radio path can be taken into account by performing channel estimation for the channels used. A known way to perform channel estimation is to generate an estimate of the impulse response of the radio channel and to equalize the received radio signals by using the received equalization data. When performing the equalization, it is essential that the antenna configuration of the array antenna and the antenna weightings remain the same as they were when the channel estimation was performed.
In the channel estimation, known signal sequences can be used. The channel estimate of user-specific channels is typically formed of a pilot sequence transmitted in a traffic channel.
In prior art, the effective power of the pilot sequences of the channels transmitted with user-specific radiation beams is low due to the restricted length of the pilot sequence and the restricted transmission power. Thus, the quality of the channel estimation can be low, which deteriorates the quality of the connection and the performance of the radio system.